1. Field of the Invention
The present invention pertains generally to the field of voltage sensors and more particularly to a voltage sensor system which utilizes the Pockels electro-optic effect to measure voltage.
2. Background Art
High-accuracy measurement of high voltage has traditionally been accomplished using iron-core ferro-magnetic potential transformers. These devices have substantially limited dynamic range, bandwidth, linearity, and electrical isolation. During electrical fault conditions these transformers can conduct dangerous levels of fault energy to downstream instrumentation and personnel, posing an additional liability.
A variety of optic sensors for measuring voltage have been developed in attempts to offer the power industry an alternative to the conventional transformer technology. Generally, these voltage sensor systems require that direct electrical contact be made with the energized conductor. This contact is made necessary by the use of a voltage divider which is utilized to connect the sensing element with the energized conductor on which a measurement is to be made. Direct electrical contact with the conductor may alter or interrupt the operation of the power system by presenting a burden or load.
In addition to the disadvantages associated with direct electrical contact with the energized conductor, prior art voltage sensor systems are typically bulky, particularly in extremely high voltage applications. This is true because the size of the voltage divider required is proportional to the voltage being measured. The size of such systems can make them difficult and expensive to install and house in substations.
Many prior art sensors are based upon the electrostrictive principle which utilize interferometric modulation principles. Unfortunately, interferometric modulation is extremely temperature sensitive. This temperature sensitivity requires controlled conditions in order to obtain accurate voltage measurements. The requirement of controlled conditions limits the usefulness of such systems and makes them unsuited for outdoor or uncontrolled applications. In addition, interferometric modulation requires a highly coherent source of electromagnetic-radiation, which is relatively expensive.
Open-air E-field based sensors have also been developed, but lack accuracy when used for measuring voltage because the open-air E-field used varies-with many noisy parameters including ambient dielectric constant, adjacent conductor voltages, moving conductive structures such as passing vehicles, and other electromagnetic noise contributions.
Systems which utilize mechanical modulation of the optical reflection within an optic fiber have also been developed. Among other drawbacks, the reliance of such systems on moving parts is a significant deterrent to widespread use.
It would therefore be an advantage in the art to provide a system which does not require direct electrical contact with the energized conductor, is compact, operates in a variety of temperatures and conditions, is reliable, and is cost effective.
It is therefore an object of the present invention to provide an electro-optic voltage sensor system which does not require contact with a conductor.
It is a further object of the present invention to provide such an electro-optic voltage sensor system which is capable of use in a variety of environmental conditions.
It is a still further object of the present invention to provide such an electro-optic voltage, sensor system which can be employed to accurately measure high voltages without use of dedicated voltage division hardware.
It is an additional object of the present invention to provide such an electro-optic voltage sensor system which minimizes the electronics needed for implementation.
It is a further object of the present invention to provide a sensor system capable of being integrated with existing types of high voltage power transmission and distribution equipment so as to reduce or eliminate the need for large stand-alone voltage measurement apparatus.
It is yet, another object of the present invention to provide a sensor system capable of being integrated with existing types of power transmission and distribution equipment.
The above objects and others not specifically recited are realized in a specific illustrative embodiment of an Electro-Optical Voltage Sensor System whereby one may measure the voltage difference (or electrical potential difference) between objects or positions. Voltage is a function of the electric field (hereinafter xe2x80x9celectric fieldxe2x80x9d shall be indicated xe2x80x9cE-fieldxe2x80x9d) and the geometries, compositions and distances of the conductive and insulating matter. Where, as in the present invention, the effects of an E-field can be observed, a voltage measurement can be calculated.
Various aspects of the invention employ a transmitter, a sensor, a detector, and a signal processor. The transmitter produces a beam of electromagnetic radiation which is routed into the sensor. Although this electromagnetic radiation used in the present invention can comprise any wavelengths beyond the, visible spectrum, the term xe2x80x9clightxe2x80x9d, will be used hereinafter to denote electromagnetic radiation for the purpose of brevity. The beam undergoes polarization before it undergoes an electro-optic effect in the transducing material of the sensor. In the polarized beam the light has at least two components (A and B) which propagate along at least two orthogonal axes, thus forming at least two orthogonal planes within the beam. The electro-optic effect occurs when the sensor is placed into an E-field, and is observable as a phase differential shift of the orthogonal beam components. The differential phase shift causes a corresponding change in the beam""s polarization, which is expressed by a shift from a circular beam of light to an elliptical beam of light which is shifted 45 degrees from the primary axes. By measuring the major and minor axes of the ellipse formed by the beam, one can determine the polarization change which has occured in the beam, which can ultimately be processed to determine the voltage.
In accordance with one aspect of the present invention, the polarized light is then passed through a 45 degree fixed phase rotator. The two components (A and B) of the beam are rotated 45 degrees and converted into a set of amplitude modulated (AM) signals of opposing polarity that are transmitted out of the sensor. The detector converts the set of optical AM signals into electrical signals from which the is voltage is determined by the signal processor. The sensor processes the beam by splitting the beam in accordance with the components (A and B) of the orthogonal polarization planes into at least two AM signals. In one embodiment, these AM signals are then converted into digital signals, fed into a digital signal processor and mathematically processed into a signal proportional to the voltage which produced the E-field.
In accordance with another aspect of the present invention, the two components of the beam (A and B) are separated and then it is passed through a 50:50 beam splitter to provide two beams of A component of equal intensity and two beams of B component of equal intensity. One of the A component beams and one of the B component beams are combined in such a way that the two beams are 180 degrees out of phase so as to determine the difference in amplitude of the different components. The other A component beams and B component beam are combined in such a way that the two beams are in phase so as to determine the sum of the amplitudes of the different components. The difference and sum of the amplitudes provide the angle of polarization caused by the E-field. The polarization caused by the E-field is proportional to the voltage carried by the monitored conductor.
Such a mechanism provides an optical solution to creating the sum and difference signals necessary for post phase rotation analysis. The to sum and difference signals are created at optical speeds and do not require fast electronics to produce. Furthermore, such a method simplifies problems which may be created by unmatched, photodiode response characteristics. Calibration between the two photodiodes now requires only simple gain changes and errors induced by changes between the transmission characteristics of the optical fibers will be reduced.